The invention relates to multiplexers for optical signals. More particularly, the invention relates to a configurable optical multiplexer having multiple channels whose operational states can be independently controlled to add, drop and pass optical signals.
Dense wave-division multiplexing (DWDM) is a technique in which multiple optical signals having different wavelengths are transmitted through a single optical fiber. DWDM enables the volume of data that can be transmitted by an individual optical fiber to be multiplied by approximately n, where n is the number of different wavelengths. Typically, networks employing DWDM are arranged with a ring topology in which several optical add-drop multiplexers (OADMs) are connected by optical fibers to form a loop. The OADMs are used to add particular optical signals to, and drop particular optical signals from, the network. The OADM may drop from the network an optical signal destined for another network element and feed such optical signals such other network element, or may add an optical signal received from another network element to a channel of the network. The other network element may be an end user, a direct or indirect connection to or from an end user, another network or a direct or indirect connection to or from another network.
Conventional add-drop multiplexers used in DWDM networks have fixed add-drop channels. Such fixed OADMs are only capable of adding or dropping an optical signal of a given frequency, and therefore are inflexible in their operation. Such inflexibility imposes limitations on the flow of traffic through the network and between the network and other network elements.
More recently, configurable optical add-drop multiplexers (COADMS) have been proposed in an effort to avoid the limitations imposed by fixed OADMs. Configurable optical add-drop multiplexers are sometimes referred to in the art as reconfigurable optical add-drop multiplexers (ROADMS). A COADM is configurable in the sense that the operational mode, i.e., drop, pass, add and add-drop, of each of its channels can be individually set by a control signal and by providing the appropriate input signal. COADMS based on optical multiplexers and 2xc3x972 optical switches are sold by JDS Uniphase Corp., for example, and are described at  less than http://www.jdsuniphase.com/HTML/catalog/products/menu_srch.cfm?-fn=coadm.html greater than . However, such COADMS do not scale well to large port counts due to insertion losses and high manufacturing costs.
COADMS based on a diffraction grating and slit have been proposed by F. N. Timofeev, P. Bayvel, E. G. Churin and J. E. Midwinter in 42 Electronics Letters, 1307-1308 (Jul. 4, 1996)). However, these COADMS suffer from several problems. They have a high insertion loss, and, although multiple optical signals can be removed from the network, only one of these optical signals can actually be transmitted into an output fiber. Finally, the optical signal is mechanically selected in this type of COADM.
U.S. Pat. No. 5,414,540 of Patel et al. (Patel) discloses a COADM capable of individually switching optical signals in an input channel or an add channel to a selected one of a drop channel or an output channel. The optical signals have to have specific directions of polarization in some of Patel""s embodiments. A diffraction grating spatially separates the optical signals in the input channel or the add channel according to their frequencies. The separated optical signals pass through different segments of a liquid-crystal modulator. The liquid-crystal modulator segments are individually controlled to rotate the polarization of the optical signal passing through them through zero or 90xc2x0. The optical signals then pass through a polarization-dispersive element, such as calcite, which spatially separates the optical signals according to their polarization. A second diffraction grating combines the optical signals having the same polarization state, i.e., rotated or not rotated, into two different, non-parallel output beams, one of which is transmitted to the output channel, the other of which passes to a drop channel.
The COADM disclosed by Patel offers a number of advantages over the optical multiplexer-based and slit-based COADMS mentioned above, but still has shortcomings. The structure of several of the embodiments disclosed by Patel causes them to suffer from an unacceptably high level of cross-talk. A COADM should not introduce cross-talk between the optical signals. When a COADM operates to drop an optical signal received at its input port, the cross-talk component of greatest concern is that between the input port and the output port. This is particularly important because it is customary for the network to include many COADMS connected in series. The cumulative effect of cross-talk into the output port in each COADM degrades the signal-to-noise ratio of the optical signals passing through the network. When a COADM operates to add an optical signal received at its add port, the cross-talk component of greatest concern is that between the add port and the drop port. These cross-talk components between the input port and the output port and between the add port and the drop port are typically coherent and in-band with the wanted signal. Worst-case calculations indicate that the specification for the cross-talk components from the add port to the drop port and from the input port and the output port should be less than xe2x88x9250 dB. That is, the power of the cross-talk optical signal divided by the power of the wanted optical signals at the output port or at the drop port should be less than 10xe2x88x925. A good reference on the crosstalk performance of optical add-drop multiplexers is E. L. Goldstein and L. Eskildsen, Scaling Limitations in Transparent Optical Networks Due to Low-level Crosstalk, 7 IEEE Photonics Tech. Lett., 95-96 (January 1995).
First, in some of the embodiments disclosed by Patel, the optical signals are incident on the elements of the COADM, including the LC modulator, at angles of incidence that differ significantly from one another and that additionally are different from zero. The polarization rotation provided by the LC modulator depends on the angle of incidence. The polarization of an optical signal will be rotated through a design angle, e.g., 90xc2x0, only at a given angle of incidence. Optical signals having other angles of incidence have their polarization rotated through angles different from the design angle. A portion of an optical signal whose polarization is not rotated through the design angle will appear as cross-talk at the other output.
Second, some of the embodiments disclosed by Patel operate with polarized optical signals having a pre-determined direction of polarization. Signal losses occur if the optical signals have directions of polarization different from the pre-determined direction, as is commonly the case of optical signals in a network. Such signal losses are undesirable in a network.
Third, the polarization-independent embodiments disclosed by Patel employ four or six half waveplates. Half waveplates not only introduce chromatic dispersion that causes cross-talk, but also need to have very accurately matched optical thicknesses to prevent them from introducing additional cross-talk. The applicants have calculated that practical manufacturing tolerances in only one of the half waveplates can result in a cross-talk level as high as xe2x88x9233 dB, which is unacceptably high for many applications, as noted above. Moreover, the effect of dispersion increases this cross-talk level significantly.
Fourth, Patel discloses an embodiment that uses a Wollaston prism to make the angles of incidence on the LC modulator all equal, but this embodiment additionally requires six half-wave plates, whose difficulties are discussed above.
Finally, the embodiments disclosed by Patel sites polarization-dispersive elements adjacent the LC modulator where the optical signals have been divided into their frequency components. This requires that the polarization-dispersive elements be relatively large, and increases the size and manufacturing cost of the COADM.
Accordingly, what is needed is a configurable optical add-drop multiplexer in which the optical signals to be added or dropped are electrically selected and that easily meets the above-mentioned xe2x88x9250 dB cross-talk specification.
What is also needed is a configurable optical add-drop multiplexer capable of operating with unpolarized optical signals or optical signals having arbitrary directions of polarization.
The invention provides a frequency-selective optical multiplexer that comprises input/output optics, an LC polarization controller and a spectral demux/mux. The input/output optics include two ports, a first optical path and a second optical path spatially separated from one another by a first distance in a first direction, and polarization-dispersive optics.
The polarization dispersive optics are disposed between the ports and the optical paths, and are structured to generate a pair of polarization components from an optical signal. The pair of polarization components is composed of a first polarization component and a second polarization component having orthogonal directions of polarization. The polarization dispersive optics are additionally structured to output the first and second polarization components via the first and second optical paths, respectively. The first and second polarization components have first and second polarization directions when the optical signal is received at one of the ports, and have the second and the first polarization directions, respectively, when the optical signal is received at the other of the ports.
The spectral demux/mux generates, from a first pair of orthogonal polarization components received from the input/output optics, first pairs of spectral components spatially separated in a direction orthogonal to the first direction, outputs the first pairs of spectral components to the LC polarization controller, receives respective second pairs of spectral components from the LC polarization controller, and spatially overlaps the second pairs of spectral components to generate a second pair of polarization components for return to the input/output optics. Either the first pair of polarization components or the second pair of polarization components passes between the spectral demux/mux and the input/output optics via the optical paths.
The LC polarization controller is located to receive the first pairs of spectral components at a zero angle of incidence, and operates to rotate the polarizations of each of the first pairs of spectral components individually and selectively through an angle of either 0xc2x0 or 90xc2x0 to generate one of the second pairs of spectral components.
The invention additionally provides a method for dropping a drop optical signal from a multi-frequency optical signal. In the method, the multi-frequency optical signal is received and is spatially separated into a first polarization component and a second polarization component having orthogonal directions of polarization. The first polarization component and the second polarization component are spatially separated into first spectral components and second spectral components, respectively. The first spectral components and the second spectral components respectively include a first drop spectral component and a second drop spectral component originating from the drop optical signal.
The polarizations of the first drop spectral component and others of the first spectral components are set to be orthogonal to one another and the polarizations of the second drop spectral component and others of the second spectral components are set to be orthogonal to one another.
The first spectral components are spatially overlapped to generate a third polarization component that includes the first drop spectral component polarized orthogonally to the others of the first spectral components. The second spectral components are spatially overlapped to generate a fourth polarization component spatially separated from the third polarization component. The fourth polarization component includes the second drop spectral component polarized orthogonally to the others of the second spectral components.
Finally, the third polarization component and the fourth polarization component are polarization-dependently spatially overlapped to generate the drop optical signal from the first drop spectral component and the second drop spectral component.
The invention finally provides a method for adding an add optical signal to a first multi-frequency optical signal to generate a second multi-frequency optical signal that includes the add optical signal. In the method, the add optical signal and the multi-frequency optical signal are received. The first multi-frequency optical signal is spatially separated into a first polarization component and a second polarization component having orthogonal polarizations. The add optical signal is spatially separated into a third polarization component polarized orthogonally to the first polarization component and a fourth polarization component polarized orthogonally to the second polarization component. The first and third polarization components are output as a fifth polarization component and the second and fourth polarization components are output as a sixth polarization component.
The fifth polarization component and the sixth polarization component are spatially separated into first spectral components and second spectral components, respectively. The first spectral components and the second spectral components respectively include a first add spectral component and a second add spectral component originating from the add optical signal.
The polarizations of the first add spectral component and others of the first spectral components are set to be parallel to one another and the polarizations of the second add spectral component and others of the second spectral components are set to be parallel to one another.
The first spectral components, including the add spectral component, are spatially overlapped to generate a seventh polarization component and the second spectral components, including the second add spectral component, are spatially overlapped to generate an eighth polarization component spatially separated from the seventh polarization component.
Finally, ones of the first spectral components having the same polarization as the first add spectral component in the seventh polarization component and ones of the second spectral components having the same polarization as the second add spectral component in the eighth polarization component are polarization-dependently spatially overlapped to generate the second multi-frequency optical signal.
The COADM according to the invention and the optical signal drop and the optical signal add methods according to the invention enable optical signals to be selectively dropped from and added to a multi-frequency optical signal using a simple electrical control signal. The COADM and the methods perform a polarization-dependent spatial separation before performing a frequency-dependent spatial separation. This enables the crosstalk levels to meet the above-described crosstalk specification.